Driver assistance systems in motor vehicles frequently use a camera system for detecting objects in the vehicle's environment, for example in an environment area located in front of the vehicle in the direction of travel. Such camera systems are typically disposed in the vehicle interior behind the windshield and look in the direction of travel through the windshield.
Some driver assistance systems, such as systems for accident prevention, e.g. (emergency) brake assistants, or systems providing longitudinal or lateral control support, e.g. speed control systems or lane assistants, need information about the vehicle's environment that is as accurate as possible. Detecting objects and determining the distance to these objects play an important part in this respect. Currently, distances to objects are primarily detected by sensors using radiation, e.g. ultrasound, radar, or LIDAR systems. But cameras are also used for determining object distances.
So-called stereo camera systems are used for determining object distances by means of cameras. These consist of two typically identical camera modules or mono cameras designed as separate optical systems, that is, they each comprise an image recording element and an imaging system (e.g. a lens). The two camera modules are mostly aligned with their optical axes in parallel and next to one another at an offset behind the windshield, that is, on a line parallel to the transverse axis of the vehicle. The lateral distance between the camera modules is called the baseline. The coordinates of the projection of one pixel of an object differ in the images captured by the two camera modules. The difference between the pairs of coordinates, that is, the offset of the pixel in the images, depends on the distance of the object from the camera. If the baseline is known, the distance to the object can be calculated from the pixel offset using known trigonometric methods. This principle of determining or calculating object distances using a stereo camera system has also been called the stereo principle.
DE 10 2010 023 591 A1, which is incorporated by reference, discloses a stereo camera system intended for installation on the windshield of a vehicle and for use by a driver assistance system. The system is formed as a mechanically coupled unit of two camera modules. Each camera module of the system includes an image recording element, such as a CMOS or CCD image chip, and a lens system, especially a single lens. The system also comprises an electronic circuit including a memory with a software for evaluating the digital image data generated by the image recording elements of the camera modules.
As described above, the stereo camera system searches for identical image contents, especially pixels, in the two images generated by the two camera modules and evaluates their offset based on the stereo principle to determine distance information. To obtain best possible results and keep the calculation effort as small as possible, the two camera modules or the two image recording elements must be aligned as precisely as possible, especially with respect to their roll angle relative to one another, that is, the rotation of the image recording elements against one another about their respective optical axes. Ideally, the pixel columns of the two image recording systems of a stereo camera system are aligned parallel to one another, i.e. the image recording elements of the camera modules do not have a roll angle or roll angle difference, respectively, relative to one another. For this purpose, the camera modules or image recording elements are currently actively adjusted during the manufacture of the stereo camera system. During the active adjustment, images are read out continuously from the modules while the camera modules are aligned. The images read out can be used to align the camera modules relative to one another.
The currently necessary adjustment of the camera modules of a stereo camera system requires a considerable effort and is cost-intensive. But leaving out the active adjustment step, that is, perform purely mechanical adjustment with sufficiently small tolerances, especially with respect to the roll angle tolerance of the image recording systems, currently fails, especially due to long tolerance chains that exist in the entire stereo camera system but also in individual components of the system, such as the individual camera modules.
The camera modules in known stereo camera systems typically include a mounting plate, e.g. a circuit board on which an image recording element is disposed. A lens system, e.g. a single lens, is located above the image recording element. Said lens may, for example, be held using a lens holder and may also be disposed directly on the mounting plate. The camera modules are manufactured separately and then arranged in the stereo camera system, especially in a common housing of the stereo camera system. The following tolerances regarding the roll angle of the image recording elements of a stereo camera system have to be taken into consideration every time:                Roll angle tolerance within the image recording element itself, i.e. in particular between the photosensitive semiconductor chip (die) and the chip package (die package)        Roll angle tolerance between image recording element and mounting plate        Tolerance between the mounting plate and the lens, especially if features (such as markers) on the lenses of the camera modules are used for roll angle alignment.        Tolerance between the lens and the lens holder        Tolerance between the camera modules or between the camera modules and the housing of the stereo camera system, respectively        
The tolerances within the camera modules of the stereo camera system must each be taken into account for both camera modules, that is, the tolerances will apply twice, which extends the tolerance chain for the entire stereo camera system.